THE COLLECTION OF MINERALS 



A chemical feature of the greatest importance in the con- 

 stitution of the silicates is the replacing power of the bases, 

 whereby one or more basic molecules take the place in whole or 

 in part of another in the chemical composition of a mineral. For 

 instance, the sesquioxide of alumina (A1 2 3 ) can be replaced by 

 the sesquioxide of iron (Fe 3 3 ), except in the feldspars; and, 

 similarly, lime (CaO), magnesia (MgO) and protoxide of iron 



CALC1TE PSEUDOMORPH AFTER ARAGONITE, ClANCIANI, ITALY 



(FeO) are substituted for one another; or any of these can take 

 the place of the oxides of potassium and sodium (K 2 0, Na 3 0). 

 This replacing process comes prominently into view in Garnet, in 

 which with the same theoretical formula for a very large group 

 the composition of the different varieties is different through the 

 effect of this law. Another, and simpler, instance is the series of 

 compounds connecting Siderite (FeC0 3 ) with Calcite (CaC0 3 ). 



Among the silicates may be mentioned Feldspar (Desk Cases 

 10 and ii, Wall Case R), Pyroxene (Desk Case n), Amphibole 



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